Lowering the temperature of the squid giant synapse prevents EPSP initiation of the postsnaptic action potential and permits investigation of the relationship between the EPSP and the presynaptic action potential. This study revealed that transmitter release is temperature sensitive; from an Arrhenius plot, an apparent activation energy of 74 kcalorie per degree, was calculated. A change in presynaptic spike height does not appear to be the major factor involved in the sensitivity of transmitter release to temperature. The effect of temperature on transmitter release is therefore likely to involve other factors such as temperature sensitivity of the release mechanism.